Electrochemical gas sensors typically comprise two or three electrodes separated by an electrolyte. These sensors generate currents in response to the presence of a gas, e.g. carbon monoxide, hydrogen sulphide, sulphur dioxide or ammonia for which they are adapted. Hitherto, faults in such sensors, for example broken signal wires or loss of electrolyte, have been detected by applying a quantity of the gas to be detected to a sensor while monitoring the sensor output. If a wire is broken or the electrolyte has leaked away, there will be no, or at least a greatly reduced, output current.
The need to test such sensors by applying quantities of gas has a number of disadvantages. Staff are required to visit each sensor which is time consuming and undesirable if a sensor is located in a clean environment such as is found in semiconductor processing plants. Also, if a sensor fails, its failure will not be detected until the next test. This of course is undesirable where the sensor is used to detect a toxic gas or an explosive gas. Furthermore, if the gas to be detected is toxic, it is undesirable that it be deliberately released during the testing process and, for domestic use in particular, this method of testing is quite unsuitable.